Leave it if you don't need to remove. Sometimes a mod will require sliding the switch down from the top if wired to a driver. In that case, the ring as to be removed. I have mods both with and without. All are fine.
I prefer the retaining ring to be present ... the stock Mag battery
spring can exert a fair amount of pressure
If you need to slide the switch from the top
you can always reinstall the retaining ring afterwards just
to be on the safe side.
I always remove them for the simple fact a KIU will not work with the ring installed, easiest way to get them out is a flat screwdriver in the middle of the ring and a good whack with a hammer and it'll come right out. I've personally always thought the ring was useless, the ground screw in a stock switch if properly tightened, will hold it into place.